Start by slicing your flank steak into thin strips, cutting against the grain for maximum tenderness.

In a small bowl, whisk together soy sauce and cornstarch until completely dissolved. Add brown sugar, rice vinegar, and sesame oil, whisking until well combined. Reserve 2 tablespoons for marinating the beef and set the rest aside for the final sauce.

Marinate the beef strips with the reserved 2 tablespoons of sauce for 15 minutes at room temperature. Meanwhile, add water to the remaining sauce ingredients and stir until smooth.

Heat your large skillet or large wok over medium-high heat with a tablespoon of oil. Sauté the bell peppers and onions for 3-5 minutes until they begin to soften, then add garlic and ginger for 1-2 minutes until fragrant. Transfer to a large bowl.

In the same pan over medium-high heat, sear the marinated beef strips until golden brown, about 2-3 minutes per side. Work in batches to avoid overcrowding, which would steam rather than sear the meat.

Add peanuts and dried red chilies to the skillet, stirring constantly for 30 seconds to 1 minute until fragrant. Return the beef and vegetables to the pan, pour the sauce over everything, and add red pepper flakes to taste. Reduce heat to medium and let everything simmer until the sauce thickens, about 3-4 minutes. The sauce should coat everything beautifully without being too thick.